Abstract

Rising costs of power generation as a result of increased condenser tube scaling and
corrosion have become critical areas that need intervention at Eskom’s power stations in
South Africa. In the past few years, the quality of cooling and raw water has deteriorated
with increased concentrations of scale formation metals such as calcium (Ca) and
magnesium (Mg) as well as Natural Organic Matter (NOM). The Dissolved Organic Matter
(DOC) fraction of NOM in the water forms complexes with metals Ca and Mg under various
conditions. In this study, the Cooling Water (CW) and Raw Water (RW) at Lethabo and
Duvha power stations were sampled and were analyzed for metals as well as organics and
the data obtained was fitted into a Visual MINTEQ chemical model and Langelier Saturation
Index (SI) calculations coupled with calcium carbonate precipitation potential models. The
data obtained indicated that as the DOC concentration increased, the SI values decreased
which may be attributed to the to complexation of Ca and Mg (the scaling metals) bound
to DOC, leaving relatively lower concentrations of the free Ca and Mg ions until the reactive
sites of DOC were saturated. The seasonal changes affect pH, DOC and concentrations of
other metals present in the water that influenced complex formation and scaling.

Keywords

Scaling Potential; Metal-organic Complexes; Visual MINTEQ

Introduction

Eskom is a major power generating industry in Southern Africa, providing more
than 95% of the electricity consumed in South Africa. In power generation, the basic
thermodynamics are governed by condensation and evaporation. In a power plant, the
cooling water is used to condense the steam by absorbing heat. The water is then cooled
down and can be re-used [1].

The water (raw and cooling (CW)) used in this process is rich in organic and inorganic
mineral ions and these metal ions cause scaling which is indicated by saturation index. The
scaling potential by these free calcium(Ca2+) and magnesium (Mg2+) is vital to Eskom’s
business as it has major cost implications. For instance, 1 mm thick scale could add 7.5%
to energy costs, 1.5 mm could add up to 15% and 7 mm thick scale could increase costs by
70% (Roberge 2008). When calculating the scaling potential of the CW it is important to
take into account the metal complexation with dissolved organic carbon as it affects the
scaling potential of the CW.

Figure 1 shows the process involved in a conventional open wet cooling system at
Eskom power stations. The cooling water system is a re-circulating system where warm
water from the condenser (where heat exchange between cooling water and steam from
the turbine takes place) passes through the cooling tower. As the water is cooled, there is
loss of water through evaporation. Raw water is then as make-up water into the cooling
tower pond to replenish water that was lost during evaporation. This water is then recycled
back into the system. The scaling reaction occurs within the condenser tubes carrying the
recycled water. It is to note that these reactions are dependent on the water composition
such as the type of metal present, size of the organic molecules and the physico-chemical
properties of water e.g. the pH and temperature [2,3,4].

Figure 1

Conventional Open Wet Cooled System at Eskom Power Stations

Recently, the humic acid concentration on H+
binding has been researched by [5].
The NICA-Donnan model for experimental data carried out by conducting acid-base
titrations with humic acid concentrations of between 20 and 200 mg/L concluding that the
concentration of humic acid in solution affects the binding behavior of the humic substance.
Some researchers explained spectroscopic and thermodynamic equilibrium calculations to
understand the complexation between Cu and NOM their research indicated that the most stable Cu—NOM complex was formed under acidic conditions due to
steric hindrances [6].

The MINTEQA2 speciation model was used in a study where metalDOM
complexes with constant metal (Ca, Mg, Cu) concentrations over
a pH range were investigated [7]. Their modeled results indicated
that in the pH range of 0-6, there was no interaction between the
Ca and DOM, but the concentration of this metal complex increased
at pH > 6 and found that the Cu complex predominates at pH range
between 2.5 and 8 and that the interaction with Mg is relatively small.

Another study by [8] reported on the competitive complexation
of metal ions with humic substances. Their results indicated that at
an increased pH, the complexation of Ni with humic acid increased
and that high concentration of Ca inhibites the complexation of Ni.

The focus of this study is to investigate the gaps of some of the
previous studies using models are EQ 3/6, Geochem, MINTEQA2, NICADonnan,
and PHREEQC and WHAM were surveyed to address the
issue of accommodating NOM in equilibrium model [9]. The objective
of the study was to understand the physico-chemical conditions under
which metals complex with DOC as well as individual metal binding
capacities. Appropriate programs will therefore provide a predictive
model that will assist to check the interplay of physicochemical
parameters and thus provide for scaling control protocols for cooling
water recycling in power generating stations.

Experimental Analysis

Sampling was done at two power stations, Lethabo and Duvha.
The raw and CW water from each station was analysed at Eskom
(RT&D) laboratories. The data was then entered into Visual MINTEQ
to further understand and model how metals complex with DOC.

pH and Alkalinity Analyses

The alkalinity of the sample was determined by electrometric titration where 25.00 ml of the sample was titrated with a
standardized solution of 0.02 N nitric acid until the end point
is reached. The Mettler Toledo MA 235 instrument (Greifensee,
Switzerland) consisting of a pH electrode with in-built temperature
compensation, was used to determine the pH of the sample.

TDS analysis

The samples were analyzed using the evaporation technique.
Here, a beaker is weighed and the mass recorded. The beaker is then
filled with the sample and the mass of the sample is recorded. The
sample is then placed in an oven and heating done at 180°C overnight.
The beaker was cooled and weighed again and TDS was calculated.

Metal analysis

The concentrations of the metal ions were determined using
inductively coupled plasma atomic emission spectrometry (ICP). The
ICP model used was a Perkin Elmer Optima, 4300 DV (Shelton, USA).
Before analysis the samples were filtered through a 0.22 µm syringe filter.

Analysis of anions

The anions in the sample were analyzed using Ion Chromatography
(IC). The IC model used was the Dionex ICS – 1500 (Sunnyvale,
California USA).

TOC and DOC

The TOC analyses were carried out on the Elementar Vario TOC
(Hanau, Germany) and for DOC, the samples were first filtered through
a 0.45 µm filter before being analyzed on the same instrument.

Flocculation experiment

A natural polyelectrolyte poly (hydroxyalkylene dimethyl
ammonium chloride) was added in various concentrations ranging
2-10 mg/L to 500 ml of raw water. Each sample mixture was then
subjected to mixing using an instrument called the Stuart Flocculator (SW6) (Stone, Staffordshire). Initially, the mixing took place for 5
mins at 200 rpm and thereafter at 80 rpm for a further 10 mins. The
samples were then allowed to stand for 1 hr before being decanted
into glass sample bottles before they were sent for analyses.

Results and Discussion

The effect of the DOC on SI using data from the Eskom
CW standard

The cooling water standard that is currently used by all stations,
does not take into account the organics in the water. Figure 2a was
obtained using Visual MINTEQ and is an indication of the effect of
DOC on the SI using data from the Eskom CW standard. As the DOC
concentration is increased, the SI decreased. This may be attributed
to the fact that as the DOC concentration increases, more of the Ca
and Mg (scaling metals) bind to the DOC leaving less of the free Ca
and Mg ions in solution that cause scaling

Figure 2A

The effect of DOC on SI using the CW standard

It has been reported and mentioned previously that based on
molecular structures, NOMs are compounds that are categorized
mainly as Humic Acids (HA) and Fulvic Acids (FA) and contain many
functional groups including carboxylic and phenolic groups. These
functional groups of NOM play an important role in binding metals
under optimized conditions of pH, temperature, DOC and metal
concentrations [10].

The effect of increasing concentrations of a poly (hydroxyalkylene
dimethyl ammonium chloride) on the % M-DOC using raw water
from Lethabo power station

This experiment was carried out to see the effect of an anionic
natural polyelectrolyte poly (hydroxyalkylene dimethyl ammonium
chloride) would have on the complexation of metals with DOC when
added at various concentrations to raw water. As the concentration
of the flocculent increased, the M-DOC concentration in the raw
water decreased Figure 2b. It is interesting to note that flocculation could only target the unbounded DOC whereas the M-DOC remains
unaffected. Among various M-DOC complexes, only %Ca-DOC
showed this decrease.

Figure 2b

The effect of various metals bound to DOC with various concentrations of an artficial anionic poly-electrolyte

Comparison of the seasonal effect on SI and the
%metal-DOC of the CW and raw water from Lethabo
and Duvha power station

Seasonal variation affects the natural atmospheric conditions
w.r.t temperature, pressure and humidity influencing concentration
of NOM and metal ions in raw and cooling water. This section provides
an insight to the seasonal effect on concertation of NOM, metal ions
and %M-DOC in the month of June-July (winter and dry season in SA)
and December (summer and rainy season in SA)

Lethabo power station

The data obtained from "Power Station A" for the CW and
RW was entered into the Visual MINTEQ programme and it was found
Figure 3a and b that the DOC in the CW decreased below 50% in the
month of December, possibly due to the rain that diluted the available
organics and metal ions. However, during June and July which is the
dry season, DOC was at a higher concentration of 70%. It is interesting
to observe that DOC concentration in raw water was not affected by
the change in season when compared to the cooling water. Further,
the metal complexation was prominent for alkaline earth metals;
therefore the focus of the study was done for calcium and magnesium
ions only. As seen in the figure, DOC concentration in raw water was
lower (6.00 – 6.5 mg/L) than that in the cooling water (55.00 – 65.00
mg/L), also the Ca2+effectively formed complexes with DOC both in
raw and cooling water when compared to Mg2+.It is interesting to
note that the complexation reaction both in cooling and raw water
were almost similar and was found to be in the range of 3.5-5 mg/L
but SI values for calcite scaling was higher for cooling water (SI : June= 1.158, July= 0.574 and December = 0.941) than for raw water (SI :
June = -0.525, July= -0.987 and December = -0.980). A positive value
of SI refers to saturation of calcite in water and can cause scale and
negative value indicate that water is unsaturated with calcite and
therefore chances of scale formation are minimal or negligible. This
difference of SI may be due to the fact that was discussed earlier as
DOC compounds have various reactive functional groups that acts as
active sites for metal complexation. Therefore, irrespective of the
higher concentrations of DOC in the water if these reactive sites are
saturated by the complexation, there is least possibility of further
reaction of metals with the DOC. From these results it is evident that
there is an optimum concentration of DOC required in the water to
form metal complexes after which no more metal complexation can
occur and the free Ca2+ ions get the best condition for the scaling
potential.

Figure 3A

The effect of %M-DOC concentration of CW and SI at Power Station A

Figure 3B

The effect of %M-DOC concentration in the RW and SI (at power station A)

Comparative analysis of raw water of Lethabo and
Duvha power stations for the winter season

"Power station B" receive the raw water supply from two
different sources i.e. north (Naaupoort dam) and south (Nooitgedaght
dam) and therefore these cooling systems were sampled. From the
experimental results obtained Table 1 and those obtained using
Visual MINTEQ Figure 3c it was observed that at "Power station A", %Ca-DOC
was found to be 4 %m/m whereas the %Mg-DOC was approximately
2.4 %m/m. However at "Power station B", the %Mg-DOC was
around 3.8 % m / m and %Ca-DOC was 3.3 %m/m. The difference
in the %Ca-DOC and %Mg-DOC could be because of the difference in
pH between the "Power station A" (7.87) and "Power station B" (North = 8.23 and South=
8.29) raw water samples. It was reported by that the speciation of
NOM under given physical conditions such pH and ionic strength
plays a huge role for M-DOC complexation [11]. Based on this fact
for the pH values, it is clear that difference in the binding capacity of Ca and Mg ions differ probably due to the NOM speciation as
evidenced by [3]. In both the cases, the SI value showed the calcite
was unsaturated to form scale.

Figure 3C

The comparison of raw water and SI from station A and station B (June sampling

Concentration (mg/L)

Station A raw

Station B raw north

Station B raw south

Alkalinity

63

104

50

Ca2+

13

37

11

Cl-

6

20

10

DOC

6

11

7

Mg2+

8

26

8

Na+

9

24

9

SO42-

14

130

21

TDS

238

355

121

pH

7.87

8.23

8.29

Table 1: Eskom laboratory results for raw water from station A and station B (June sampling)

Comparative analysis of cooling water of North and
South Duvha power station during winter (June and
July) and summer seasons (December)

This section deals with the results obtained for the cooling
water from the north and south side of "Power station B" that was sampled in
the month of June, July and December. The data w.r.t pH, DOC, Ca
and Mg ions indicated that the change in the season affected the
concentration of DOC as well as the Ca2+ and Mg2+ ions. Even
though the concentration of Ca2+, Mg2+ and DOC varied, the ratio in
which these metals were complexed with DOC was similar Figure 4a at more or less similar pH of CW. It was interesting to note that
Mg2+ effectively bonded to DOC in all the season when compared to
Ca2+-DOC complexation as the pH 8-9 allowed Mg2+ to bond with
DOC probably by deprotonation of the –COOH and –OH groups of
DOC [12]. The concentration Ca2+ ions for all the season can easily be
correlated to corresponding SI values.

Figure 4a

The %M-DOC and SI in the CW from power station B (NorthThe %M-DOC and SI in the CW from power station B (NorthThe %M-DOC and SI in the CW from power station B (North

Figure 4b shows data obtained from Eskom laboratories for
CW from the southern side of "Power station B" for which the
pH of the water varied only slightly. The results indicated that the
higher percentage of Ca-DOC and Mg-DOC was found in June when
compared to July. The free Ca2+ was 250 mg/L which was the highest
for July that could be directly affected the SI value of 1.128 and CaDOC
was 1.4 (%m/m)

Figure 4b

The %M-DOC and SI in the cooling water from power station B (South)

Conclusion

The results from this study clearly indicate that the concentration
of organics and its speciation in the water play an important role for
scaling potential by Ca2+ ions. The seasonal variation influences the
pH, concentration for divalent ions and DOC and that further affects
the degree of complexation in raw and cooling water. The availability
of free Ca2+ does not influence complex formation with DOC which
depends upon the pH of water that change with season but directly
affects the SI

Acknowledgments

This research was supported by Eskom RT&D and Eskom TESP
program for running cost of the project. NRF is acknowledged for
tuition fee to register at University of Johannesburg

References

Reeves JA. Chemical Speciation Modelling of the Cooling Water
Circuit at Matla Power Station. MSc thesis, University of Kwa
Zulu Natal, Durban, South Africa. 1997.
(Ref)